History: The German Spy Ring

Major Vernon Kell, the first head of the Security Service, masterminded an operation that culminated in the round-up of Germany's spy network in Britain on 4 August 1914 - the day of the outbreak of the First World War.

German Intelligence had made considerable efforts before the war to place agents in Britain. The Germans sought to spy particularly on the Royal Navy's ships and installations, and had produced some useful intelligence for the Kaiser's naval staff.

Nevertheless, over 20 German agents had been identified and dealt with by Kell, or "K" as he was known, in the years between his appointment in 1909 and the summer of 1914. This was achieved in spite of the fact that in April 1914 he had only fourteen staff working for him in the Secret Service Bureau.

The extent of Kell's success stemmed from a conversation that one of his officers, Stanley Clarke, had overheard on a train in 1911. Clarke heard two Germans talking about a strange letter that one of them had received from Potsdam asking questions about British preparations for war. The letter's recipient was the proprietor of the Peacock Hotel in Leith, and when approached by Clarke he provided full details of the mysterious correspondence.

The author of the letter, a "Mrs Reimers", was in fact a senior member of the German Intelligence Service named Gustav Steinhauer. Kell was granted permission to intercept this and other correspondence, from which a picture emerged of a ring of agents and intelligence "post boxes".

This interception technique relied on great secrecy and, later on, much skill in developing without detection the scientific means of reading German messages written in invisible ink and concealed in innocent-looking documents. Many of MI5's key successes during the war were owed to the development of this technique.

Springing the trap

On the evening of 3 August 1914, the Secret Service Bureau received a secret message that war against Germany was to be declared the following day. Contingency plans for such an eventuality had already been put in place. An ongoing investigation of a man named Karl Gustav Ernst had earlier revealed the names of a number of suspected spies, as subsequently described by Sir Basil Thomson, the head of Scotland Yard's Special Branch during the war:

"[Ernst] was the central post office for all German espionage papers [in the UK]. All he had to do for his pittance of 20 [shillings] a month was to drop the letters he received, ready stamped with English postage stamps, into the nearest pillar-box. In this way the names and addresses of no less than 22 spies were compiled, and the arrangements were so complete that on the very day of the declaration of war 21 of the 22 agents were arrested and interned simultaneously, and one only evaded arrest by embarking for Germany."
(The Times, 12 November 1921)

The arrested suspects were based in a variety of locations - from Falmouth in the west, to Barrow and Newcastle in the north, as well as London. When their cases were fully investigated, eight of those detained were found to have played no hostile intelligence role. The remaining thirteen were indeed found to be German agents. They were interned indefinitely or tried.

The arrests wrecked German efforts to establish a regular stream of intelligence from Britain, just at the moment when it was most needed. Kell later spoke of his determination at the time "to paralyse the German espionage effort in one powerful blow". Afterwards, German Intelligence could only send poorly trained agents to Britain, who were quickly caught.

Britain's vital naval secrets would have been at particular risk, had the agents not been captured. The British Army may also have had cause to celebrate the detection of the German spy network. Kell himself later claimed that as late as 21 August, the German commanders were still unaware that the British Expeditionary Force had been sent to France, although this was common knowledge to thousands of people in Britain.

• Security Service files from this period were released to the Public Record Office, now the National Archives (new window), in 1997 - file references KV 1/39, 40 and 41.

Karl Gustav Ernst - hairdresser and spy

Karl Gustav Ernst was an Islington hairdresser of German descent, though he was a British citizen. He was one of the suspected spies detained on 4 August 1914, and had been under investigation since October 1911.

Interception of his mail had revealed that he had been in regular contact with a German Secret Service officer known by the pseudonym of "Mrs Tony Reimers". The Germans were particularly interested in the activities of the "Intelligence Department of the War Office"and wanted to be kept informed of ongoing espionage cases.

Ernst was also accused of communicating"certain information relating to the movements, armaments and dispositions of ships of the Royal Navy, which was calculated to be useful to an enemy." He pleaded not guilty but was found guilty of espionage and sentenced to seven years' hard labour.

• More information on this case can be found in the National Archives (new window) in the file CRIM 1/151/2.

How German spies used invisible ink

Invisible ink is a substance which can be used to write hidden messages on an otherwise innocent-looking document. A number of different chemicals can be used to produce invisible writing. The hidden message can be restored by means such as heating or treatment with a particular chemical.

The document shown on the left was intercepted in July 1915 in a letter to Norway. Its discovery led to the arrest of Courtney de Rysbach, a music hall entertainer who was found to be a German spy.

Investigators found that the invisible text had been written between the lines of a musical score.Its author had used a secret ink prepared from toothpaste impregnated with potassium ferrocyanide. This type of invisible ink can be revealed by being treated with a solution of ferric chloride, which makes the text appear as blue ink.

The secret message reads:

"If you could only send me some money I could get my brother who is in the Navy to give me all the Navy movements, he could be most valuable (but) he would want money for it. I expect in 2 weeks to be an interpreter at least I have good hopes and then can give you much information, like I am doing now is very difficult. The people know nothing and they say very little, one must be in the thing one's self then you can get good and valuable information.

I will send you a telegram with my new name and address please telegraph me the money it is safer! The newspaper trick is found out and very carefully watched".

Cecil.

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